The proposed research seeks to determine: (1) the importance for mental health of shifts in the timing of circadian rhythms, and (2) the physiological basis for control of rate of circadian phase shifts. Physiological and behavioral rhythms of about 24 hours in length (equal circadian rhythms) in men and other mammals require about 7 days to shift and entrain to a reversed daily schedule in response to environmental stimuli (such as to light in the case of the laboratory rat). We will investigate the sensory and physiological bases of our recent discoveries that: (1) pinealectomy leads to an accelerated phase shift in young to subadult rats and a retarded rate in older ones, and (2) surgical interruption of the dural membrane can lead to abnormal entrainment endpoints.